U.S. patent number 7,848,452 [Application Number 11/660,358] was granted by the patent office on 2010-12-07 for distortion compensating apparatus.
This patent grant is currently assigned to Mitsubishi Electric Corporation. Invention is credited to Nobuhiko Ando, Ryoji Hayashi.
United States Patent |
7,848,452 |
Hayashi , et al. |
December 7, 2010 |
Distortion compensating apparatus
Abstract
A distortion compensating apparatus has an IQ imbalance
compensation coefficient arithmetic circuit 31 for calculating,
from correlation values of a transmission signal and a feedback
signal, IQ imbalance compensation coefficients for compensating for
IQ imbalance in a quadrature modulator 23; and an IQ imbalance
compensator 21 for compensating for the IQ imbalance with respect
to a distortion compensated signal output from a distortion
compensator 11 according to the IQ imbalance compensation
coefficients, and for outputting to the quadrature demodulator
23.
Inventors: |
Hayashi; Ryoji (Tokyo,
JP), Ando; Nobuhiko (Tokyo, JP) |
Assignee: |
Mitsubishi Electric Corporation
(Tokyo, JP)
|
Family
ID: |
36059748 |
Appl.
No.: |
11/660,358 |
Filed: |
September 13, 2004 |
PCT
Filed: |
September 13, 2004 |
PCT No.: |
PCT/JP2004/013320 |
371(c)(1),(2),(4) Date: |
February 16, 2007 |
PCT
Pub. No.: |
WO2006/030481 |
PCT
Pub. Date: |
March 23, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070286307 A1 |
Dec 13, 2007 |
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Current U.S.
Class: |
375/297;
375/298 |
Current CPC
Class: |
H03F
1/3247 (20130101); H03F 1/3294 (20130101); H04L
27/364 (20130101); H03F 2200/336 (20130101); H04L
2027/0016 (20130101) |
Current International
Class: |
H04L
27/36 (20060101) |
Field of
Search: |
;375/296-298,302 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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06-268703 |
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Sep 1994 |
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JP |
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2001-016283 |
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Jan 2001 |
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JP |
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3144649 |
|
Jan 2001 |
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JP |
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2001-168774 |
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Jun 2001 |
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JP |
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2002-077285 |
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Mar 2002 |
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JP |
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WO-03/081867 |
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Oct 2003 |
|
WO |
|
Other References
Koh Ri, Takeharu et al., "Automatic Self Correction Circuit using
TDMA-TDD" STE Telecommunication Engineering Co., Ltd. and Casio
Computer Co., Ltd., B-314 IEICE Spring Conference 1993. Publication
Date: Mar. 15, 1993. cited by other .
James K. Cavers, "The Effect of Quadrature Modulator and
Demodulator Errors on Adaptive Digital Predistorters for Amplifier
Linearization," IEEE Transactions on Vehicular Technology, vol. 46,
No. 2, pp. 456-466, May 1997. cited by other.
|
Primary Examiner: Lugo; David B
Attorney, Agent or Firm: Birch, Stewart, Kolasch &
Birch, LLP
Claims
What is claimed is:
1. A distortion compensating apparatus comprising: a distortion
compensator for compensating for distortion caused by a power
amplifier according to distortion compensation coefficients that
are stored in a distortion compensation coefficient memory in
connection with a transmission signal and that correspond to
amplitude of the transmission signal; a quadrature modulator for
carrying out quadrature modulation of a distortion compensated
signal; a power amplifier for power amplifying a quadrature
modulated signal; a quadrature demodulator for carrying out
quadrature demodulation of a power amplified signal and for
outputting a feedback signal; a distortion compensation coefficient
arithmetic circuit for calculating, from the transmission signal
and the feedback signal, distortion compensation coefficients
corresponding to the amplitude of the transmission signal, and for
updating distortion compensation coefficients stored in the
distortion compensation coefficient memory; an IQ imbalance
compensation coefficient arithmetic circuit for calculating, from
correlation values of the transmission signal and the feedback
signal, IQ imbalance compensation coefficients in real time for
compensating for IQ imbalance in said quadrature modulator; and an
IQ imbalance compensator for compensating for the IQ imbalance with
respect to the distortion compensated signal output from said
distortion compensator according to the IQ imbalance compensation
coefficients obtained in real time, and for outputting an IQ
imbalance compensated signal to said quadrature modulator.
2. The distortion compensating apparatus according to claim 1,
further comprising: a delay circuit for adjusting timing of the
transmission signal and the feedback signal; and an evaluation
function arithmetic circuit for obtaining a delay of said delay
circuit from the transmission signal and the feedback signal, and
sets the delay into said delay circuit, wherein processing by said
evaluation function arithmetic circuit is followed by processing by
said IQ imbalance compensation coefficient arithmetic circuit and
processing by said IQ imbalance compensator.
3. The distortion compensating apparatus according to claim 2,
which repeatedly carries out the processing by said evaluation
function arithmetic circuit, the processing by said IQ imbalance
compensation coefficient arithmetic circuit and the processing by
said IQ imbalance compensator.
4. The distortion compensating apparatus according to claim 2,
wherein said evaluation function arithmetic circuit obtains the
delay of said delay circuit using, as evaluation function,
cross-correlation or error sum of squares between the transmission
signal and the feedback signal.
5. The distortion compensating apparatus according to claim 4,
wherein at a delay at which the timing of transmission signal and
the timing of the feedback signal matches, said evaluation function
becomes maximum when said evaluation function arithmetic circuit
uses cross-correlation between the transmission signal and the
feedback signal as the evaluation function.
6. The distortion compensating apparatus according to claim 4,
wherein at a delay at which the timing of transmission signal and
the timing of the feedback signal matches, said evaluation function
becomes minimum when said evaluation function arithmetic circuit
uses error sum of squares between the transmission signal and the
feedback signal as the evaluation function.
7. A distortion compensating method comprising: compensating for
distortion caused by a power amplifier according to distortion
compensation coefficients that are stored in a distortion
compensation coefficient memory in connection with a transmission
signal and that correspond to amplitude of the transmission signal;
outputting a distortion compensated signal; carrying out, by
utilizing a quadrature modulator, quadrature modulation of said
distortion compensated signal and for outputting a quadrature
modulated signal; power amplifying said quadrature modulated
signal; carrying out, by utilizing a quadrature demodulator,
quadrature demodulation of a power amplified signal and for
outputting a feedback signal; calculating, from the transmission
signal and the feedback signal, distortion compensation
coefficients corresponding to the amplitude of the transmission
signal, and for updating distortion compensation coefficients
stored in the distortion compensation coefficient memory;
calculating, from correlation values of the transmission signal and
the feedback signal, IQ imbalance compensation coefficients in real
time for compensating for IQ imbalance in said quadrature
modulator; compensating for the IQ imbalance with respect to the
outputted distortion compensated signal according to the IQ
imbalance compensation coefficients obtained in real time; and
outputting an IQ imbalance compensated signal to said quadrature
modulator.
8. The distortion compensating method according to claim 7, further
comprising: adjusting, by utilizing a delay circuit, timing of the
transmission signal and the feedback signal; and obtaining a delay
of said delay circuit from the transmission signal and the feedback
signal; and setting the delay into said delay circuit.
9. The distortion compensating method according to claim 8, wherein
obtaining the delay of said delay circuit using, as evaluation
function, cross-correlation or error sum of squares between the
transmission signal and the feedback signal.
10. The distortion compensating method according to claim 9,
wherein at a delay at which the timing of transmission signal and
the timing of the feedback signal matches, said evaluation function
becomes maximum when cross-correlation between the transmission
signal and the feedback signal is used as the evaluation
function.
11. The distortion compensating method according to claim 9,
wherein at a delay at which the timing of transmission signal and
the timing of the feedback signal matches, said evaluation function
becomes minimum when error sum of squares between the transmission
signal and the feedback signal is used as the evaluation function.
Description
TECHNICAL FIELD
The present invention relates to a distortion compensating
apparatus for compensating for nonlinear distortion of a power
amplifier for amplifying a transmission signal.
BACKGROUND ART
As shown in Patent Document 1, for example, a conventional
distortion compensating apparatus has a problem in that its
distortion compensation performance deteriorates because of the
gain error and phase shift error of the .pi./2 phase shifter
between the in-phase (I) channel component and quadrature (Q)
channel component in a quadrature modulation circuit or quadrature
demodulation circuit. Here, the gain error and the phase shift
error are collectively called IQ imbalance.
In contrast with this, the Patent Document 1 circuit, in addition
to the conventional distortion compensation circuit, compensates
for the IQ imbalance by causing a waveform generating circuit to
output a predetermined baseband signal, and by installing an error
measuring circuit for measuring a demodulation error by extracting
a demodulation signal corresponding to the baseband signal, and an
error correcting circuit for eliminating the demodulation error
from the demodulation signal.
In addition, although not describing the distortion compensating
apparatus, Patent Document 2 and Non-Patent Document 1 each
describes a technique for compensating for the IQ imbalance
involved in the quadrature modulation.
The Patent Document 2 circuit compensates for the IQ imbalance by
linearly converting the in-phase amplitude signal and quadrature
amplitude signal with a linear converting means; by extracting a
level signal by supplying a level signal generating means with a
modulation wave output of the quadrature modulator to which the
linearly converted signal is input; and by causing a parameter
generating means to derive linear converting parameters for the
level signal to be used for the linear conversion.
Although the first embodiment of the Patent Document 2 obtains the
linear converting parameters from the level signal using a known
test signal, the second embodiment obtains the linear converting
parameters from the level signal in accordance with an in-phase and
quadrature amplitude input. More specifically, instead of using the
special test signal, the second embodiment calculates the linear
converting parameters for the compensation by using particular
values of a transmission signal such as values at the moment when
the I component or Q component becomes zero, or when the I
component becomes equal to the Q component.
Furthermore, the Non-Patent Document 1 describes an idea of
adjusting the IQ imbalance of a quadrature modulation circuit and a
quadrature demodulation circuit by using a loopback signal.
However, it does not describe a concrete error detecting method or
adjusting method.
Patent Document 1: Japanese patent application laid-open No.
6-268703/1994 (Paragraphs 0014 and 0019).
Patent Document 2: Japanese patent No. 3144649 (Paragraphs
0026-0028 and 0065-0068).
Non-Patent Document 1: Kohri, Yamamoto, Kawanaka, "Study of
Automatic Adjusting Circuit Using TDMA-TDD" Proceedings of the 1993
IEICE Spring Conference, Mar. 15, 1993, No. 2, B-314.
It is necessary for the conventional distortion compensating
apparatus using the technique of the Patent Document 1 or of the
first embodiment of the Patent Document 2 to transmit the
particular test signal to determine the IQ imbalance compensation
coefficients (the linear converting parameters of the Patent
Document 2) for compensating for the IQ imbalance. However, mobile
communication or broadcasting base station equipment to which the
distortion compensating apparatus is applied cannot transmit the
particular test signal after their operation has been started
because they operate continuously all day long. Thus, the IQ
imbalance is compensated for by obtaining the IQ imbalance
compensation coefficients only once by using the test signal before
starting the operation after installing the base station.
Accordingly, the distortion compensating apparatus has a problem of
being unable to cope with temperature changes or aging phenomena
after starting the operation, thereby deteriorating the distortion
compensation performance.
In addition, it is necessary for the distortion compensating
apparatus utilizing the technique of the second embodiment of the
conventional Patent Document 2 to use the particular values of the
transmission signal to determine the IQ imbalance compensation
coefficient. However, there is no guarantee that the transmission
signal with the specified particular values appear during the
actual operation, and even if it appears it takes a long time
before the appearance because the specified values are very rare
and the probability of appearing is low. Thus, the distortion
compensating apparatus has a problem of taking a long time before
compensating for the IQ imbalance.
The present invention is implemented to solve the foregoing
problems. Therefore it is an object of the present invention to
provide a distortion compensating apparatus capable of obtaining
the IQ imbalance compensation coefficients in real time using any
given transmission signal, and capable of preventing the
deterioration in the distortion compensation performance in spite
of the temperature changes or aging after starting the
operation.
DISCLOSURE OF THE INVENTION
A distortion compensating apparatus in accordance with the present
invention includes a distortion compensator for compensating for
distortion caused by a power amplifier mentioned below according to
distortion compensation coefficients that are stored in a
distortion compensation coefficient memory in connection with a
transmission signal and that correspond to amplitude of the
transmission signal; a quadrature modulator for carrying out
quadrature modulation of a distortion compensated signal; a power
amplifier for power amplifying a quadrature modulated signal; a
quadrature demodulator for carrying out quadrature demodulation of
a power amplified signal and for outputting a feedback signal; a
distortion compensation coefficient arithmetic circuit for
calculating, from the transmission signal and the feedback signal,
distortion compensation coefficients corresponding to the amplitude
of the transmission signal, and for updating distortion
compensation coefficients stored in the distortion compensation
coefficient memory; an IQ imbalance compensation coefficient
arithmetic circuit for calculating, from correlation values of the
transmission signal and the feedback signal, IQ imbalance
compensation coefficients for compensating for IQ imbalance in the
quadrature modulator; and an IQ imbalance compensator for
compensating for the IQ imbalance with respect to the distortion
compensated signal output from the distortion compensator according
to the IQ imbalance compensation coefficients, and for outputting
to the quadrature demodulator.
The present invention can obtain the IQ imbalance compensation
coefficients in real time using any given transmission signal.
Consequently the present invention offers an advantage of being
able to compensate for the IQ imbalance using the transmission
signal in operation in spite of the temperature changes or aging
after starting the operation, and to prevent the deterioration in
the distortion compensation performance.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a configuration of the distortion
compensating apparatus of an embodiment 1 in accordance with the
present invention;
FIG. 2 is a block diagram showing a configuration of the distortion
compensating apparatus of an embodiment 2 in accordance with the
present invention; and
FIG. 3 is a flowchart illustrating a flow of the timing adjusting
processing between the transmission signal and feedback signal and
the IQ imbalance compensation processing in the distortion
compensating apparatus of the embodiment 2 in accordance with the
present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
The best mode for carrying out the invention will now be described
with reference to the accompanying drawings to explain the present
invention in more detail.
Embodiment 1
FIG. 1 is a block diagram showing a configuration of the distortion
compensating apparatus of an embodiment 1 in accordance with the
present invention. The distortion compensating apparatus has a
distortion compensation controller 10, an IQ imbalance compensator
21, a D/A (digital/analog) converter 22, a quadrature modulator 23,
a local oscillator 24, a power amplifier 25, a directional coupler
26, a quadrature demodulator 27, a local oscillator 28, an A/D
(analog/digital) converter 29, a correlator 30 and an IQ imbalance
compensation coefficient arithmetic circuit 31.
The distortion compensation controller 10 has a distortion
compensator 11, an absolute value arithmetic circuit 12, delay
circuits 13 and 14, a subtracter 15, a distortion compensation
coefficient arithmetic circuit 16, and a distortion compensation
coefficient memory 17.
Next, the operation will be described.
The distortion compensator 11 of the distortion compensation
controller 10 compensates for the distortion generated by the power
amplifier 25 by performing the complex multiplication of the
transmission signal x(nT) (n is a sampling number and T is a
sampling period) and distortion compensation coefficients
corresponding to the amplitude |x(nT)| of the transmission signal
x(nT), which are stored in the distortion compensation coefficient
memory 17, thereby outputting a distortion compensated signal. The
IQ imbalance compensator 21 compensates for the IQ imbalance by
performing the complex multiplication of the distortion compensated
signal from the distortion compensator 11 and the IQ imbalance
compensation coefficients for compensating for the IQ imbalance
computed by the IQ imbalance compensation coefficient arithmetic
circuit 31. Thus, the IQ imbalance compensator 21 compensates for
the gain error and the phase shift error of the .pi./2 phase
shifter between the in-phase (I) channel component and quadrature
(Q) channel component in the quadrature modulator 23, and outputs
an IQ imbalance compensated signal y(nT). The D/A converter 22
carries out the D/A conversion of the IQ imbalance compensated
signal y(nT) output from the IQ imbalance compensator 21, thereby
outputting a complex baseband signal.
The quadrature modulator 23 carries out the quadrature modulation
of the complex baseband signal output from the D/A converter 22
with the oscillation signal from the local oscillator 24. The power
amplifier 25 performs the power amplification of the quadrature
modulated signal fed from the quadrature modulator 23. During the
power amplification, nonlinear distortion occurs. The directional
coupler 26 supplies the signal amplified by the power amplifier 25
to an antenna not shown, extracts part of the signal and supplies
it to the quadrature demodulator 27. The quadrature demodulator 27
carries out the quadrature demodulation of the signal fed from the
directional coupler 26 with the oscillation signal from the local
oscillator 28. The A/D converter 29 performs the A/D conversion of
the quadrature demodulated signal output from the quadrature
demodulator 27 and outputs a feedback signal z(nT).
The absolute value arithmetic circuit 12 of the distortion
compensation controller 10 calculates the amplitude |x(nT)| of the
transmission signal x(nT). When calculating, by using the error
signal e(nT), the distortion compensation coefficients
corresponding to the amplitude |x(nT)| of the transmission signal
x(nT) calculated by the absolute value arithmetic circuit 12, the
delay circuit 13 sets the timing in such a manner as to write them
to the address of the distortion compensation coefficient memory 17
corresponding to the amplitude |x(nT)| of the transmission signal
x(nT). The delay circuit 14 provides the transmission signal x(nT)
with the delay corresponding to the duration during which the
transmission signal x(nT) becomes the feedback signal z(nT) through
the distortion compensator 11, IQ imbalance compensator 21, D/A
converter 22, quadrature modulator 23, power amplifier 25,
directional coupler 26, quadrature demodulator 27, and A/D
converter 29, thereby adjusting the timing between the feedback
signal z(nT) and the transmission signal x(nT).
The subtracter 15 of the distortion compensation controller 10
subtracts the transmission signal x(nT) whose timing is adjusted by
the delay circuit 14 from the feedback signal z(nT) output from the
A/D converter 29, and outputs the error signal e(nT). Using the
error signal e(nT) output from the subtracter 15, the distortion
compensation coefficient arithmetic circuit 16 calculates the
distortion compensation coefficients, and updates the distortion
compensation coefficients stored in the distortion compensation
coefficient memory 17. The distortion compensation coefficient
memory 17 writes the distortion compensation coefficients that are
calculated and updated by the distortion compensation coefficient
arithmetic circuit 16 into the address corresponding to the
amplitude |x(nT)| of the transmission signal x(nT), and reads the
distortion compensation coefficients corresponding to the amplitude
|x(nT)| of the transmission signal x(nT). The distortion
compensator 11 compensates for the distortion caused by the power
amplifier 25 by performing complex multiplication of the
transmission signal x(nT) and the distortion compensation
coefficients corresponding to the amplitude |x(nT)| of the
transmission signal x(nT) read from the distortion compensation
coefficient memory 17, and outputs the distortion compensated
signal.
The correlator 30 calculates correlation values of the real part
x.sub.I(nT) and imaginary part x.sub.Q(nT) of the transmission
signal x(nT) and the real part z.sub.I(nT) and imaginary part
z.sub.Q(nT) of the feedback signal z(nT). The IQ imbalance
compensation coefficient arithmetic circuit 31 calculates the IQ
imbalance compensation coefficients for compensating for the IQ
imbalance from the correlation values calculated by the correlator
30, that is, the IQ imbalance compensation coefficients for
compensating for the gain error and the phase shift error of the
.pi./2 phase shifter between the in-phase (I) channel component and
quadrature (Q) channel component in the quadrature modulator
23.
Next, the compensation processing of the IQ imbalance will be
described in more detail.
Let us assume here that the distortion caused by the power
amplifier 25 is negligibly small. Then the initial value of the
distortion compensation coefficients stored in the distortion
compensation coefficient memory 17 is one. In addition, when the IQ
imbalance is not compensated, the initial value of the IQ imbalance
compensation coefficients calculated by the IQ imbalance
compensation coefficient arithmetic circuit 31 is also one. In this
case, the output signal y(nT) of the IQ imbalance compensator 21 is
equal to the transmission signal x(nT) as shown by the following
expressions (1) and (2). y.sub.I(nT)=x.sub.I(nT) (1)
y.sub.Q(nT)=x.sub.Q(nT) (2)
Assume in the quadrature modulator 23 that the gain error, which is
given by the ratio between the voltage gain of the in-phase (I)
channel component and that of the quadrature (Q) channel component,
is .alpha.; and that the phase shift error of the .pi./2 phase
shifter is .epsilon.. Then the output of the quadrature modulator
23 is given by the following expression (3). y.sub.I(nT) cos
n.omega.T-.alpha.y.sub.Q(nT) sin (n.omega.T+.epsilon.) (3) where
.omega. is the angular frequency of the oscillation signal of the
local oscillator 24. As for the output of the quadrature modulator
23 given by the foregoing expression (3), after being amplified by
the power amplifier 25, its part is extracted by the directional
coupler 26.
Assume that the directional coupler 26 gives a phase rotation
.delta.. Then the signal input to the quadrature demodulator 27 is
given by the following expression (4), where a is the voltage gain
of the power amplifier 25 and directional coupler 26 in
combination. ay.sub.I(nT) cos
(n.omega.T+.delta.)-a.alpha.y.sub.Q(nT) sin
(n.omega.T+.epsilon.+.delta.) (4)
The quadrature demodulator 27 multiplies the signal given by the
foregoing expression (4) by the local oscillation signal fed from
the local oscillator 28 and the local oscillation signal passing
through the .pi./2 phase shifter, and the low-pass filter removes
the second harmonic of the local oscillation signal. Assume that
the quadrature demodulator 27 has no IQ imbalance, then its output
converted by the A/D converter 29, that is, the feedback signal
z(nT) is given by the following expressions (5) and (6).
z.sub.I(nT)=ay.sub.I(nT) cos .delta.-a.alpha.y.sub.Q(nT) sin
(.epsilon.+.delta.) (5) z.sub.Q(nT)=ay.sub.I(nT) sin
.delta.+a.alpha.y.sub.Q(nT) sin (.epsilon.+.delta.) (6) Thus, the
following expressions (7) and (8) are obtained from the foregoing
expressions (1) and (2). z.sub.I(nT)=ax.sub.I(nT) cos
.delta.-a.alpha.x.sub.Q(nT) sin (.epsilon.+.delta.) (7)
z.sub.Q(nT)=ax.sub.I(nT) sin .delta.+a.alpha.x.sub.Q(nT) cos
(.epsilon.+.delta.) (8)
The correlator 30 calculates autocorrelations A, B and C of
x.sub.I(nT) and x.sub.Q(nT) by the following expressions (9)-(11),
and cross-correlations D, E, F and G of x.sub.I(nT), x.sub.Q(nT),
z.sub.I(nT) and z.sub.Q(nT) by the following expressions
(12)-(15).
.times..times..function..times..times..function..times..function..times..-
times..function..times..times..function..times..function..times..times..fu-
nction..times..function..times..times..function..times..function..times..t-
imes..function..times..function. ##EQU00001##
The IQ imbalance compensation coefficient arithmetic circuit 31
calculates the IQ imbalance compensation coefficients .alpha., sin
.epsilon., a and sin .delta. by the following expressions (20)-(23)
by calculating the following expressions (16)-(19) from the
foregoing expressions (9)-(15), and by solving the expressions
(16)-(19). D=aA cos .delta.-a.alpha.B sin (.epsilon.+.delta.) (16)
E=aA sin .delta.+a.alpha.B cos (.epsilon.+.delta.) (17) F=aB cos
.delta.-a.alpha.C sin (.epsilon.+.delta.) (18) G=aB sin
.delta.+a.alpha.C cos (.epsilon.+.delta.) (19)
.alpha..times..times..times..times..times..times..times..times..delta.
##EQU00002##
In particular, when transmitting a multiplex signal of OFDM or CDMA
as the transmission signal x(nT), the real part x.sub.I(nT) and
imaginary part x.sub.Q(nT) of the transmission signal x(nT) are
considered to have no correlation. Thus, it is possible to place as
follows. B=0 (24) In this case, the foregoing expressions (20)-(23)
become the following expressions (25)-(28).
.alpha..times..times..times..times..times..times..times..delta.
##EQU00003##
The IQ imbalance compensator 21 solves the foregoing expressions
(7) and (8) in terms of x.sub.I(nT) and x.sub.Q(nT), and gives the
following expressions (29) and (30).
.function..function..times..times..times..delta..function..times..functio-
n..delta..times..times..times..times..function..function..times..times..ti-
mes..delta..function..times..times..times..delta..times..times..alpha..tim-
es..times..times..times. ##EQU00004## The foregoing expressions
(29) and (30) give the inverse function which indicates the values
of the transmission signal x(nT) when the feedback signal z(nT) is
given.
Then, the IQ imbalance compensator 21 carries out the complex
multiplication of the transmission signal given by the foregoing
expressions (29) and (30) and the IQ imbalance compensation
coefficients calculated by the IQ imbalance compensation
coefficient arithmetic circuit 31 using expressions (20)-(23) or
expressions (25)-(28), converts the transmission signals
x.sub.I(nT) and x.sub.Q(nT) to y.sub.I(nT) and y.sub.Q(nT) as shown
by the following expressions (31) and (32), and outputs them.
.function..times..function..times..function..delta..function..times..func-
tion..delta..times..times..times..times..times..times..times..times..times-
..times..function..times..function..function..times..function..times..time-
s..times..delta..function..times..times..times..delta..times..times..alpha-
..times..times..times..times..times..times..times..times..times..times..fu-
nction..times..function. ##EQU00005## In particular, when there is
no correlation between the real part x.sub.I(nT) and imaginary part
x.sub.Q(nT) of the transmission signal x(nT), the following
expressions hold.
.function..function..function. ##EQU00006##
.function..function..function. ##EQU00006.2##
When the IQ imbalance compensator 21 outputs y.sub.I(nT) and
y.sub.Q(nT) given by the foregoing expressions (31) and (32), the
A/D converter 29 outputs the feedback signals z.sub.I(nT) and
z.sub.Q(nT) given by the following expressions (33) and (34), which
are obtained by substituting the foregoing expressions (31) and
(32) into the foregoing expressions (5) and (6).
z.sub.I(nT)=x.sub.I(nT) (33) z.sub.Q(nT)=x.sub.Q(nT) (34)
The foregoing expressions (33) and (34) mean that the IQ imbalance
compensator 21, outputting y.sub.I(nT) and y.sub.Q(nT) given by the
foregoing expressions (31) and (32), compensates for the IQ
imbalance in the quadrature modulator 23. In this way, it can
prevent the deterioration in the distortion compensation
performance due to the IQ imbalance.
As described above, the present embodiment 1 can obtain the IQ
imbalance compensation coefficients in real time using any given
transmission signal x(nT) by obtaining the correlation between the
any given transmission signal x(nT) and its feedback signal z(nT).
Accordingly, the present embodiment 1 offers an advantage of being
able to compensate for the IQ imbalance using the current
transmission signal x(nT) in spite of the temperature changes or
aging after starting the operation, and to prevent the
deterioration in the distortion compensation performance.
Embodiment 2
The foregoing embodiment 1 assumes that the timings between the
feedback signal z(nT) and transmission signal x(nT) are adjusted by
the delay circuits 13 and 14. Even in the case where the timings
between the feedback signal z(nT) and transmission signal x(nT) are
off, and in addition, the IQ imbalance occurs, the present
embodiment 2 carries out the IQ imbalance compensation after making
timing adjustment.
FIG. 2 is a block diagram showing a configuration of the distortion
compensating apparatus of an embodiment 2 in accordance with the
present invention, which includes an evaluation function arithmetic
circuit 32 in addition to the foregoing embodiment 1 shown in FIG.
1. In FIG. 2, since the remaining components are the same as those
of FIG. 1, they are designated by the same reference numerals and
their description will be omitted here.
Next the operation will be described.
The evaluation function arithmetic circuit 32, varying the delay
.tau. of the transmission signal x(nT), calculates the evaluation
function S(.tau.) of the transmission signal x(nT) and feedback
signal z(nT) output from the A/D converter 29, and sets the delay
.tau. at which the evaluation function S(.tau.) becomes maximum or
minimum to the delay circuits 13 and 14.
Here, the evaluation function arithmetic circuit 32 uses as the
evaluation function S(.tau.) the cross-correlation or error sum of
squares of the amplitude |x(nT)| of the transmission signal x(nT)
and the amplitude |z(nT)| of the feedback signal z(nT) given by the
following expressions (35) and (36), for example.
.function..tau..times..times..function..times..function..tau..function..t-
au..times..times..function..function..tau. ##EQU00007##
Assume that the distortion of the power amplifier 25 is small
enough and the quadrature modulator 23 and quadrature demodulator
27 have no IQ imbalance. In this case, at the delay .tau. at which
the timing of feedback signal z(nT) matches the timing of the
transmission signal x(nT), the evaluation function S(.tau.) becomes
maximum when it is the cross-correlation, and the evaluation
function S(.tau.) becomes minimum when it is the error sum of
squares. The evaluation function arithmetic circuit 32 sets the
delay corresponding to the delay .tau. thus obtained to the delay
circuits 13 and 14, thereby enabling the timing adjustment.
When the IQ imbalance is negligible, the timing adjustment is
completed. However, when the IQ imbalance is not negligible, the
delay obtained deviates from the correct delay because of the
effect of the IQ imbalance.
FIG. 3 is a flowchart illustrating a flow of the timing adjusting
processing between the transmission signal and feedback signal and
the IQ imbalance compensation processing in the distortion
compensating apparatus of the embodiment 2 in accordance with the
present invention. In FIG. 3, steps ST11-ST17 show the calculation
of the evaluation function S(.tau.) and the timing adjusting
processing by the evaluation function arithmetic circuit 32, and
step ST18-ST20 show compensation processing of the IQ imbalance,
which is the same as that of the embodiment 1.
At step ST11, the evaluation function arithmetic circuit 32 sets
the delay .tau. of the transmission signal x(nT) at zero; and sets
the register of the evaluation function S0 at zero when using the
cross-correlation as the evaluation function, and at M (M is a
sufficiently large value) when using the error sum of squares as
the evaluation function. At step ST12, the evaluation function
arithmetic circuit 32 calculates the evaluation function S(.tau.)
from the transmission signal x(nT) and the feedback signal
z(nT+.tau.) fed from the A/D converter 29.
At step ST13, the evaluation function arithmetic circuit 32 makes a
decision as to whether S(.tau.) calculated at step ST12 is greater
than S0 (=0) set at step ST11 when using the cross-correlation as
the evaluation function, and as to whether S(.tau.) calculated at
step ST12 is smaller than S0 (=M) set at step ST11 when using the
error sum of squares as the evaluation function.
When the decision result at step ST13 is YES, the evaluation
function arithmetic circuit 32 sets S(.tau.) calculated at step
ST12 into the register of the evaluation function S0 at step ST14,
and sets the delay .tau. at that time into the register of delay
.tau.0. On the other hand, when the decision result at step ST13 is
NO, the evaluation function arithmetic circuit 32 skips the
processing of step ST14, and proceeds to the processing of step
ST15.
At step ST15, the evaluation function arithmetic circuit 32
increments the delay .tau. of the transmission signal x(nT) by
.DELTA..tau.. At step ST16, the evaluation function arithmetic
circuit 32 makes a decision as to whether the delay .tau. is less
than a predetermined maximum delay .tau.max, and when .tau. is less
than .tau.max, it repeats the processing from step ST12 to step
ST15. When .tau. exceeds .tau.max at step ST16, the evaluation
function arithmetic circuit 32 sets the value of delay .tau., which
is set in the register of delay .tau.0 at step ST14, into the delay
circuits 13 and 14 at step ST17.
At step ST18, the correlator 30 calculates the correlations A, B,
C, D, E, F and G from the transmission signal x(nT) and feedback
signal z(nT+.tau.) according to the foregoing expressions (9)-(15).
At step ST19, the IQ imbalance compensation coefficient arithmetic
circuit 31 calculates the IQ imbalance compensation coefficients
.alpha., sin .epsilon., a and sin .delta. from the correlations A,
B, C, D, E, F and G obtained at step ST18 using the foregoing
expressions (20)-(23) or the foregoing expressions (25)-(28). At
step ST20, the IQ imbalance compensator 21 compensates for the IQ
imbalance by carrying out the complex multiplication of the
transmission signal fed from the distortion compensator 11 and the
IQ imbalance compensation coefficients .alpha., sin .epsilon., a
and sin .delta. calculated at step ST19.
If the foregoing processing cannot complete the compensation for
the IQ imbalance, the processing from step ST11 to step ST20 is
repeated at step ST21 by a predetermined number of times as
needed.
As described above, even if there is a deviation between the timing
of transmission signal x(nT) and that of the feedback signal z(nT)
and there is the IQ imbalance, the present embodiment 2 can obtain
the IQ imbalance compensation coefficients in real time using any
given transmission signal x(nT) by obtaining the correlation
between the any given transmission signal x(nT) and its feedback
signal z(nT) after adjusting the timing of the transmission signal
x(nT) and feedback signal z(nT) by obtaining the delay that will
maximize or minimize the evaluation function such as the
cross-correlation or error sum of squares of the any given
transmission signal x(nT) and feedback signal z(nT). Accordingly,
the present embodiment 2 offers an advantage of being able to carry
out the timing adjustment and compensate for the IQ imbalance using
the current transmission signal x(nT) in spite of the temperature
changes or aging after starting the operation, and to prevent the
deterioration in the distortion compensation performance.
Although the foregoing embodiments 1 and 2 have the IQ imbalance
compensator 21 between the distortion compensator 11 and the D/A
converter 22 so that the IQ imbalance compensator 21 carries out
the IQ imbalance compensation processing in digital processing,
this is not essential. For example, the IQ imbalance compensator 21
can be interposed between the D/A converter 22 and the quadrature
modulator 23 in order to carry out the IQ imbalance compensation
processing in an analog mode after the output of the IQ imbalance
compensation coefficient arithmetic circuit 31 undergoes the D/A
conversion.
INDUSTRIAL APPLICABILITY
As described above, the distortion compensating apparatus in
accordance with the present invention can obtain the IQ imbalance
compensation coefficients in real time, and can compensate for the
IQ imbalance using the current transmission signal x(nT) in spite
of the temperature changes or aging after starting the operation.
Thus, it is suitable for equipment that needs preventing the
deterioration in the distortion compensation performance.
* * * * *